Membrane bioreactor waste water treatment method

ABSTRACT

The invention relates to a membrane bioreactor waste water treatment method comprising a sludge dewatering step and a step involving the recycling of the liquid effluent originating from the sludge dewatering step, which is performed at the head of the bioreactor. The invention is characterised in that: the biological sludge extracted from the bioreactor ( 1 ) is brought into contact with the liquid effluent from the sludge-dewatering step such that the residual polyelectrolyte content in said effluent, which was used for sludge conditioning during the dewatering step ( 4 ), moves towards the biological sludge; the biological sludge is separated from the liquid effluent such as to produce a polyelectrolyte-free liquid effluent and a polyelectrolyte-loaded biological sludge; the polyelectrolyte-free liquid effluent is recycled at the head of the membrane bioreactor, and the polyelectrolyte-loaded biological sludge is conveyed towards the dewatering step.

The present invention relates to the treatment of waste water using membrane bioreactors.

It is known that during such treatments, it is necessary to dewater the biological sludge from the reactor, this sludge previously undergoing a chemical conditioning to ensure its flocculation. Polymers and, more generally, polyelectrolytes are used for this conditioning, particularly in order to obtain bulky, well-differentiated flocs, in a clarified interstitial water. The liquid effluent from the dewatering operation is recycled to the head of the membrane bioreactor.

The return of this effluent to the head of the membrane bioreactor incurs a major risk due to the fact that this effluent, produced by the sludge dewatering operation, contains relatively large residual quantities of polyelectrolytes that are liable to cause severe, indeed irreversible, clogging of the bioreactor membranes.

During the treatment of waste water using a membrane bioreactor, the daily recycle rate at the head, that is of the liquid effluent from the dewatering operations, routinely represents 1 to 5% of the daily urban waste water feed rate and sometimes more than 10% of the daily rate when industrial waste water is treated.

When the sludge dewatering treatment and the recycling of the liquid effluent from this dewatering to the head of the bioreactor are carried out in batch mode, which is often the case, the proportion between the effluent from the dewatering and the waste water feed to the bioreactor may occasionally be much higher, thereby further aggravating the risk of clogging the membranes of the water treatment system, that is of the membrane bioreactor.

To overcome this drawback, two alternatives are proposed today by a person skilled in the art:

-   -   1) Avoid the risk of clogging. In this case, the recycling of         the liquid effluent to the head is prohibited. The treatment of         the sludge from the membrane bioreactor is then transferred to a         nearby conventional station. In fact, it is not systematically         possible to install such a sludge treatment station near the         waste water treatment station and, in any case, this solution         implies the transport of volumes of sludge that may be difficult         to accept if the station is large.     -   2) Control the risk:         -   a) by minimizing the quantity (that is the batching) of             polyelectrolytes used for conditioning the sludge subjected             to the dewatering treatment, by making sure to reintroduce             the recycled liquid effluent from the sludge dewatering             treatment to the head of the bioreactor, at the point             furthest from the bioreactor membranes, and to spread these             recyclings to the head over time, in order to guarantee the             greatest possible dilution with the waste water fed to the             membrane bioreactor. This may in particular result in the             need to provide a buffer tank temporarily storing the             effluent before its recycling to the head.             -   This alternative can help to manage the risk of clogging                 the bioreactor membranes, but it does not eliminate the                 risk of an accidental overbatching of the                 polyelectrolytes during the sludge treatment. It is                 especially difficult to control such a risk because the                 quantitative analysis of the residual polyelectrolytes                 present in the head recyclings is technically complex,                 indeed impossible today.         -   b) by subjecting the effluents from the sludge dewatering             treatment to a pretreatment in order to destroy the residual             quantities of polyelectrolytes. However, this solution             presents the drawback of being very costly because the             removal of a few milligrams per liter of residual quantities             of polyelectrolytes usually involves the at least partial             removal of the pollution from the effluent produced by             sludge dewatering. Thus, for example, an oxidation treatment             by ozone, of the residual quantities of polyelectrolytes,             implies a very large and uneconomic batching due to the             ozone demand of the effluent (oxidation of organic matter).             More generally, the oxidizing treatments carried out on such             effluents can also give rise to oxidation by-products that             are difficult to remove by the water system if the latter is             not designed to treat this type of induced pollution.

Finally, in the absence of a genuine “physical barrier”, the pretreatments proposed for the time being do not guarantee the total elimination of the risk of clogging the membranes of the water treatment system.

On the assumption that these membranes are seriously clogged by the polyelectrolyte, a person skilled in the art can apply chemical washing procedures to restore the performance of the membranes. However, the effectiveness of these procedures is haphazard and the chemicals they employ are aggressive to the membranes, jeopardizing their service life. Furthermore, these procedures involve costly large-scale maintenance and the immobilization of a portion of the membrane area that is then no longer available for waste water filtration. This drawback results in the need to oversize the membrane filtration portion.

It is therefore an object of the present invention to permit the recycling, to the head of the water system of a membrane bioreactor waste water treatment installation, of the effluent from the sludge system, that is from the dewatering step of the excess sludge produced by the water system, while completely eliminating the risk of clogging the membranes of the water system by the residual polyelectrolytes from the sludge system.

In consequence, this invention relates to a method for treating waste water by a membrane bioreactor comprising a sludge dewatering step and a recycling, to the head of the bioreactor, of the liquid effluent from the sludge dewatering, characterized in that:

-   -   the biological sludge extracted from the bioreactor is contacted         with the liquid effluent from the sludge dewatering step, so         that the residual content, in said effluent, of polyelectrolyte         used to condition the sludge during the dewatering step moves         toward the biological sludge;     -   the biological sludge is separated from the liquid effluent so         as to produce a polyelectrolyte-free liquid effluent, on the one         hand, and a polyelectrolyte-loaded biological sludge, on the         other hand;     -   the polyelectrolyte-free liquid effluent is recycled to the head         of the membrane bioreactor and,     -   the polyelectrolyte-loaded biological sludge is sent to the         dewatering step.

According to the present invention, the biological sludge is separated from the liquid effluent by filtration on microfiltration or ultrafiltration membranes. By selecting a cutoff threshold that is significantly lower than the molecular weight of the polyelectrolytes used during the sludge dewatering, the membranes of the sludge system act as a genuine physical barrier and produce an effluent free of traces of polyelectrolytes, which can be recycled to the water system, that is to the head of the bioreactor, without the risk of clogging the membranes thereof, the polyelectrolyte-loaded biological sludge or flocs being sent to the sludge dewatering step.

Other features and advantages of the present invention will appear from the description given below with reference to the drawings appended hereto, which illustrate a non-limiting embodiment thereof. In the drawings:

FIG. 1 is a diagrammatic view of an installation employing the method according to the present invention, and

FIG. 2 is a similar view to FIG. 1, showing an embodiment of the method according to the invention.

FIG. 1 shows at 1 the water system for treating urban or industrial waste water, this system comprising a membrane bioreactor. This installation comprises a dewatering unit for the sludge produced by the water system, designated as a whole by the numeral 4, with a polyelectrolyte injection for conditioning the sludge, this unit producing dewatered sludge, on the one hand, and a dewatering effluent removed via a line 6, on the other hand.

The excess sludge from the water system is contacted with the dewatering effluent conveyed via the line 6, in a contactor 2. During this contact, the residual quantity of polyelectrolyte in the dewatering effluent moves from the liquid phase to the biological flocs or sludge produced by the water system, by adsorption, absorption and flocculation mechanisms at levels very far from saturation. The polyelectrolyte fixation efficiency is close to 100%.

Thus the method according to the present invention permits the intimate contacting of a large quantity of excess biological sludge extracted from the membrane bioreactor (concentration about 8 to 12 g/l) with a small quantity of residual polyelectrolyte present in the dewatering effluent. The polyelectrolyte losses in the dewatering unit 4 may represent nearly 10% of the initial batching, indeed more in case of overbatching.

The contactor 2 may be, , for example, a “TURBACTOR” rapid mixing reactor as described in Memento Technique de l'Eau, volume 2, page 638 published by Degremont (50^(th) anniversary edition 1989).

The biological sludge is then separated from the liquid phase by filtration in order to produce a polyelectrolyte-free liquid effluent, recycled to the head of the water system via a line 7, and a polyelectrolyte-loaded biological sludge which is conveyed to the dewatering unit 4 via the line 5. As mentioned above, this separation can be carried out by filtration on microfiltration or ultrafiltration membranes, acting as a physical barrier in order to produce an effluent free of traces of polyelectrolyte. This effluent can therefore be recycled to the water system without the risk of clogging the membranes thereof. The membranes of the sludge system may have any geometry (plane, tubular, hollow fiber), may be made of any appropriate material (organic or ceramic) and may present any operating configuration (external system or immersed system).

FIG. 2 shows an embodiment of the method according to the invention. In this FIG. 2, the characteristic values of the installation used during this application and the data obtained are mentioned.

An examination of this figure shows that the “leakage” of polyelectrolyte in the sludge dewatering unit 4 represents 200 g/day. This residual polyelectrolyte is transferred, in the contactor 2, on 200 kg of biological sludge produced by the water system 1, representing a ratio of 1000.

The surface area of the membranes 3 (60 m²) installed in the sludge system only accounts for 3% of the area of the membranes (2000 m²) of the bioreactor of the water system. This means that the risk of accidentally clogging the membranes is limited to 3% of the total membrane area of the treatment installation. Furthermore, the dissociation of the membranes of the water system 1 and the membranes 3 of the sludge system serves to carry out a chemical washing of the membranes of the sludge system without reducing the filtration capacity of the water system. It is also possible to provide two sets of membranes on the sludge system (one in operation, the other in washing or on standby), considering the small surface area of the membranes of this sludge system; this is obviously impossible, for economic reasons, on the water system, because of the large surface area of the bioreactor membranes.

This figure shows that the dewatering effluent recycled to the head of the water system is free of polyelectrolyte, thereby eliminating any risk of clogging the membranes of the water system.

Furthermore, the invention serves to reduce the consumption of polyelectrolyte used for conditioning the sludge in the dewatering unit 4. In fact, the batching of polyelectrolyte necessary for dewatering the sludge produced by the water system is 10 kg/TMeS. The recycling of the “leaks” of polyelectrolyte to the dewatering unit (line 5) serves to reduce the consumption of polyelectrolyte by 10%, said consumption thereby being 1.8 kg/day instead of 2 kg/day.

The advantages provided by the present invention are in particular the following:

-   -   it serves to control the clogging character of the liquid         effluents produced by the sludge dewatering step, by         transferring a small quantity of polyelectrolyte to a large         quantity of biological flocs (ratio 500 to 4000);     -   it guarantees the absence of residual traces of polyelectrolyte         in the liquid effluent recycled to the head of the water system,         thereby eliminating the risk of clogging the membranes of this         system;     -   it serves to reduce the batching of polyelectrolyte during         sludge dewatering thanks to the recycling of the leaks of         polyelectrolyte on the sludge from the water system, before it         is conditioned.

It remains obvious that the present invention is not limited to the embodiments described and shown above, but encompasses all variants thereof. 

1. A method for treating waste water by a membrane bioreactor comprising a sludge dewatering step and a recycling, to the head of the bioreactor, of the liquid effluent from the sludge dewatering, characterized in that: the biological sludge extracted from the bioreactor (1) is contacted with the liquid effluent from the sludge dewatering step, so that the residual content, in said effluent, of polyelectrolyte used to condition the sludge during the dewatering step (4) moves toward the biological sludge; the biological sludge is separated from the liquid effluent so as to produce a polyelectrolyte-free liquid effluent, on the one hand, and a polyelectrolyte-loaded biological sludge, on the other hand; the polyelectrolyte-free liquid effluent is recycled to the head of the membrane bioreactor, and the polyelectrolyte-loaded biological sludge is sent to the dewatering step.
 2. The method as claimed in claim 1, characterized in that the biological sludge is separated from the liquid effluent by filtration on microfiltration or ultrafiltration membranes (3), the cutoff threshold of said membranes being significantly lower than the molecular weight of the polyelectrolytes used during the sludge dewatering. 